Traditionally, nonwoven webs used as components of disposable absorbent articles have been formed of fibers spun from polymer resins, frequently resins including polyolefins such as polypropylene. While such resins may be used to form fibers that are flexible and smooth and have a soft tactile feel and desirable mechanical properties, they are derived from petroleum. In recent years, demand has developed for alternative materials that are more quickly biodegradable and/or are derived from resources deemed to be renewable.
One such material that is currently receiving interest as an alternative is polylactic acid (PLA). PLA is a thermoplastic aliphatic polyester derived from resources such as corn starch or sugarcane. It has been found that fibers spun from PLA generally exhibit less elongation at break and significantly greater modulus than fibers of comparable size or denier spun from polyolefins such as polypropylene.
It is possible to manufacture a nonwoven web from fibers spun entirely from PLA resin. While the comparatively greater modulus of PLA fibers may impart comparatively greater tensile strength to a nonwoven web formed of them, fibers formed of currently available PLA resins tend to be comparatively less flexible and have a rough tactile feel. Thus, a nonwoven web formed of entirely PLA fibers may be more stiff and rough-feeling, as compared with a nonwoven web formed of polyolefin (e.g., polypropylene) fibers of similar size/denier and basis weight. This may make a PLA fiber nonwoven web undesirable for applications in which flexibility, tactile softness and smoothness are desirable, such as, for example, to form components of wearable absorbent articles such as disposable diapers, absorbent pants, feminine hygiene pads and other articles. In such articles, tactile softness may be desired because nonwoven web components often contact (and sometimes rub) the skin; and tactile softness and and flexibility may be desired for wearer comfort.
It has been further found, however, that core-sheath configuration bicomponent fibers may be spun using PLA to form the core section and a polyolefin (e.g. polypropylene) to form the sheath section. The polyolefin sheath section can give the fiber tactile surface feel characteristic of all-polyolefin fibers, while the PLA core section can maintain or even enhance fiber tensile strength, and replaces a portion of the petroleum-derived component with a component derived from resources deemed to be renewable.
In any process involving the production of polymer fiber nonwoven web or the production of articles in which the nonwoven web is used as a component, there will be an associated production of scrap such as edge trimming scrap, cutoff scrap, material that is not useable or not saleable because it does not meet specifications, etc. It is typical for such scrap to be recycled into the spinning process to help reduce the production of waste, and help control material costs. A nonwoven web containing polyolefin and PLA components together, however, presents technical complications for recycling because polyolefin and PLA are generally immiscible with each other, and thus, tend to form separate phases in melt. If spinning a melted blend of these resins is attempted, problems may be presented including inconsistently shaped fibers, weakened or broken fibers, resin accumulation and “drip” at the spinnerets, and others that may deleteriously affect web quality, consistency and mechanical properties.
Accordingly, there is a need for ways in which materials including combinations of PLA and polyolefins such as polypropylene can be successfully recycled in a fiber spinning/nonwoven web manufacturing operation.